Troubleshooting Your AR Build
At this point I have completed hundreds of AR builds from standard .223, 5.56 Nato, .223 Wylde, 5.45x39, 9mm, 10mm, and even a few in the 308 and 7.62x51 chambers. I will say none have gone together perfectly however a few tips and tricks along the way have helped me get a bit smarter with each build. Experience does breed success and thus the reason I generally believe a defensive firearm you will bet your life on should be built by a professional. Following are a number of tips and tricks which in sum should resolve or prevent the vast majority of AR format functioning issues.
UNDERSTANDING THE ZEN STATE OF AR FUNCTIONING
The AR format is by its nature a much more complex machine with more design similarities to an engine than a bolt action rifle. The gas pressure operated system can be a bit finicky and can also be the main cause of problems. Including myself, many an expert has professed the reliability improvements of heavy buffer springs, carriers, and buffers, however now with more experience, I have seen that these are just band aid solutions to an over-gassed or poorly designed gas system or poorly tuned rifle. Other times, problem resolution can be as simple as just swapping out a magazine or part, so lets take a look at some best practices to avoid problems in the first place.
A lot happens as the trigger is pulled, the hammer drops, hits the firing pin which sets off the primer and charge. The pressure in the cartridge pushes the bullet down the barrel and after the bullet passes the gas port, gas is leached off and blown back into the bolt via the gas block, gas tube, and gas key on the carrier. Pressure increases and pushes on the gas seals in the bolt which unlocks the bolt and cycles the action. A lot can go wrong in that entire process.
The AR format is by its nature a much more complex machine with more design similarities to an engine than a bolt action rifle. The gas pressure operated system can be a bit finicky and can also be the main cause of problems. Including myself, many an expert has professed the reliability improvements of heavy buffer springs, carriers, and buffers, however now with more experience, I have seen that these are just band aid solutions to an over-gassed or poorly designed gas system or poorly tuned rifle. Other times, problem resolution can be as simple as just swapping out a magazine or part, so lets take a look at some best practices to avoid problems in the first place.
A lot happens as the trigger is pulled, the hammer drops, hits the firing pin which sets off the primer and charge. The pressure in the cartridge pushes the bullet down the barrel and after the bullet passes the gas port, gas is leached off and blown back into the bolt via the gas block, gas tube, and gas key on the carrier. Pressure increases and pushes on the gas seals in the bolt which unlocks the bolt and cycles the action. A lot can go wrong in that entire process.
Always keep in mind that the AR15 was designed to run with a 20” barrel with a rifle length gas system and full rifle length buffer tube, spring and longer & heavier buffer. By moving to a shorter carbine gas system and buffer tube assembly we are not exactly helping improve performance, recoil, or reliability but instead making it harder for the system actually do its job in a shorter overall system. The shorter the overall rifle system, the harder it is to tune for reliable operation.
DIRECT IMPINGEMENT SYSTEM VS. PISTON
For a new AR builder, I would stay away from piston systems until you have some build experience or purchase a complete working upper or complete factory rifle. Superlative Arms makes incredible system that is amazingly reliable, however for the less experienced builder a gas impingement system. For the scope of this article, I will focus on standard AR15 style direct Impingement gas system rifles as they are far more popular for the masses.
For a new AR builder, I would stay away from piston systems until you have some build experience or purchase a complete working upper or complete factory rifle. Superlative Arms makes incredible system that is amazingly reliable, however for the less experienced builder a gas impingement system. For the scope of this article, I will focus on standard AR15 style direct Impingement gas system rifles as they are far more popular for the masses.
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| A typical builders kit without a stripped lower |
GAS ISSUES
One of the most common causes of problems on the AR format are those related to the gas system. Too much gas ends up battering both you and shortening component life, but too little affects functional reliability with a rifle that sometimes cycles.
One of the most common causes of problems on the AR format are those related to the gas system. Too much gas ends up battering both you and shortening component life, but too little affects functional reliability with a rifle that sometimes cycles.
Too Much Gas - An overgassed/high gas pressure driven system is generally balanced out by heavier components at the breech end to slow the cycling down. The result is that we upgrade from a standard buffer to a heavy “H” marked buffer, heavier “H2”, and even very heavy “H3” buffers and marginally heavier M16 bolt carriers. There are even heavy specialty buffer springs available. All we have done is increase the reciprocating mass to the point where the gun cycles properly and less violently. Sure the heavier components balance things out, however too much hot gas means more overall stress on all components and more dirty hot gas in the action which equates to a hotter and dirtier gun; not to mention an overall heavier gun in the process.
Cause - The cause is usually an oversized barrel gas port diameter due to a “better safe than sorry” manufacturing model of most barrel companies. The thought process is that too much gas pressure will always function, to little could cause functioning issues with certain components.
Solution - A solution is the above heavier BCG, buffer spring, and/or buffer, however the best solution is to control the gas pressure in the first place with an adjustable gas block such as those from Superlative Arms. I stand by the comment that this is the single best upgrade you can make to an AR15. Recoil, muzzle rise, component wear, and filth all go down and the rifle runs a little cooler and cleaner. Once you are able to control the gas pressure, you can do some really cool things like further reduce gas pressure when using a lightweight BCG from YM or JP Rifles or others with a standard buffer all without affecting reliability.
Adjustable gas block are the key is to bring things in balance. See my Adjustable Gas block article here for tuning how-to. Basically keep reducing gas pressure with your lightest load until the bolt does not lock back and then increase the pressure setting by a quarter turn. The need for a heavy bolt carrier is instantly myth busted once your are able to adjust the gas pressure and balance the system.
Too Little Gas - This will cause short stroking, double feeds, failures to feed/extract/lock-back, and stovepipes.
Cause - System diagnosed as under-gased are usually the result of improper assembly, however binding components, too heavy of carriers, springs, or buffers or other design issues can exaggerate or imitate an under-gased system. There are certainly fixes which do overcome many design or functional issues caused when fielded, however addressing the root cause is the best route to a durably reliable AR.
Solution - In almost all cases the gas block is not pushing full pressure. Usually the gas block is not aligned to the gas port in the barrel or has been mounted crocked. Sometimes it is canted or has moved from gas pressure and/or has not been secured properly. In some situations I have seen out of spec gas blocks which can leak pressure.
Usually the perfect gas block placement is not slammed against the turned edge on the barrel, but sometimes it is. Drawing a center-line down the barrel with a pencil can help. Disassembly and reassembly with a set of calipers to measure where the hole is on the barrel in relation to the hole on the gas block. Remember usually the rearward most retainer screw on the gas block aligns with the hole in the gas block. One trick during assembly is to install the gas block upside down before installing the gas tube to find the exact positioning of the gas block from the shoulder on the barrel.
BOLT CARRIER PROBLEMS
Smooth Movement - An easy quick check after each cleaning is recommended. Occasionally you will have a BCG that binds for one reason or another which definitely affects functioning. Point your unloaded AR upper skyway and bounce the bolt up and down in the receiver a couple time to simulate cycling. This simple little test will tell you if something is binding. Rarely you might have an out of spec carrier, or loose gas key, but it could also be some junk or a partial brass case stuck in the receiver. Clean it out and replace parts as required.
Bad Seals - Rarely an issue. Those little seals on the bolt do wear out eventually or can become aligned in such a way that they will leak. Assure all the cuts are not aligned and/or replace as necessary.
Ejector Tension - Generally a problem that will have even the experienced guys in the crowd throwing their hands in the air and selling off a great gun. Brass should eject at about 2-3 o’clock and about three feet from the side of the gun. If they are hurling 10+ feet away and you start to have feeding/functioning/ejection issues about halfway through the mag, then an over-tensioned ejector is probably the issue. The military went to a higher-tension/stronger spring to improve full-auto fire functioning with M16 carriers, however in most cases too strong of an ejector spring will not allow a round to properly feed on the bolt face and/or from a timing perspective cause the cases to attempt to eject before they clear the port. The result more often than not are double feeds or failures to fully or partially eject. Most people think they need a stronger spring if they see this and it is usually quite the opposite.
Solution - If the ejector cannot be pressed in slightly with the fingernail into the bolt, then it is probably too tight. Another test is to remove the bolt and place an empty case in the extractor lip and lever it down on to the ejector; in some cases I have seen that the ejector is so strong that the extractor cannot hold the brass in place. Remove the spring and either grind it down or swap to a shorter ejector spring. I know the DPMS 308’s have two different ejectors; one version puts way too much tension on the same spring length. The general rule is that if you cannot re-install the ejector without using a tool to push it in, then it is too strong.
Solution - If the ejector cannot be pressed in slightly with the fingernail into the bolt, then it is probably too tight. Another test is to remove the bolt and place an empty case in the extractor lip and lever it down on to the ejector; in some cases I have seen that the ejector is so strong that the extractor cannot hold the brass in place. Remove the spring and either grind it down or swap to a shorter ejector spring. I know the DPMS 308’s have two different ejectors; one version puts way too much tension on the same spring length. The general rule is that if you cannot re-install the ejector without using a tool to push it in, then it is too strong.
Loose/Leaky Gas Key - Rarely an issue. Gas Keys do come loose rarely. Inspect the carrier end-to-end during cleaning. If it is loose, use permanent Loc-tite (Permatex gasket sealer is Mil-Spec) and tighten it back down.Bolt Bounce - This is a situation where the bolt bounces slightly after it chambers a round because the bolt is moving too fast due to excessive gas pressure and/or to strong of a buffer spring. Without super high speed video equipment, this almost impossible to witness with the naked eye, however it can occassionally be experienced during really fast shooting. This is experienced in the form of a failure to fire with the hammer dropping. Basically it is a timing issue when the bolt is bouncing out of battery at the moment the hammer drops and the firing pin hits air or a partially chambered round. If you are seeing bulged cases at the based or nice heavy primer strikes on the primers and then a light strike and failure to fire during high rate firing, then you may have a bolt bounce issue. The solution is to move to a heavier carrier or buffer or reduce the buffer spring tension or gas pressure level with an adjustable gas block. Generally the adjustable gas block is the easiest and least expensive solution.
Bolt Not Locking Back - Check the gas block alignment and assure there are no gas tube obstructions. Stick the straw of a WD-40 can down the gastube and if it comes out the barrel roughly the same rate as your are spraying it then you are OK in most cases. What to watch for is WD-40 sprayed in one end and it dripping slowly out the other. If so check the alignment and gas tube. For the most part, AR15s, especially carbines are seriously over gased to the point most will run even if half the tube or port is covered. The most common problem is that the gas block is not even close to being properly aligned with the barrel's gas port. Although rare, sometimes a builder will have the rear vent hole sealed on the tube and.or have used a 308 buffer and buffer spring or rifle length buffer and/or spring in a carbine or an H2 or H3 buffer which can retard the bolt so much that it cannot fully cycle or lock back. See the buffer weights and measures data below to the right buffer components.One part which has given me problems is actually the bolt catch. I have installed and tossed a couple out-of-spec loose bolt catches. If the bolt catch is not decently tight, I have had issues with them not catching a bolt on occasion.
POST ASSEMBLY INSPECTION
This may seem like an obvious step, however I have caught more issues in this step than any other just with close examination of an empty unloaded AR all under the assumption that “Stuff works loose.”
This may seem like an obvious step, however I have caught more issues in this step than any other just with close examination of an empty unloaded AR all under the assumption that “Stuff works loose.”
Assure the flash-hider, buffer tube castle nut are both tight and secured with Loc-Tite. Are all the pins and detent present, does the trigger function correctly even when holding the trigger down and hand cycling the bolt to imitate the next round cycling? When you point the muzzle skyware on a separated upper receiver, does the bolt float effortlessly without bind as you bouncing it up and down into the chamber by hand?
SELECTING PARTS
Typically builders get all emotional about what they want based on whatever their buddy or the latest expert recommended, however the reality is that most of the components do not increase reliability, but some do such as an adjustable gas block. Stick with Mil-Spec components if you are focused on reliability as high tolerance “3-Gun” performance parts may throw off the operational balance of an AR without a little tuning/tweaking.
Typically builders get all emotional about what they want based on whatever their buddy or the latest expert recommended, however the reality is that most of the components do not increase reliability, but some do such as an adjustable gas block. Stick with Mil-Spec components if you are focused on reliability as high tolerance “3-Gun” performance parts may throw off the operational balance of an AR without a little tuning/tweaking.
In general every upper or lower receiver I have ever tested has functioned perfectly. The upper or lower receivers have never been a root cause of any issue I have ever experienced, nor the buffer tube, gas tube, stock parts kit, or the furniture, so choose what you like. Barrels for the most part have not delivered issues themselves regardless of chambering or the finish of the feed ramps, however gas port diameters do vary which can create gas pressure issues. From my experience, pressure problems related to the barrel and gas block have been responsible for well over 95% of my functional issues. The other 5% of issues can usually be tracked to the buffer spring and buffer rates and weights.
STANDARD BUFFER & SPRINGS WEIGHTS & MEASURES (all values approximate)
AR15 RIfle - Buffer 5.2oz, 5.905” - Spring 44 coils, uncompressed 12.75”
AR15 Carbine - Buffer 3.0oz, 3.25” - Spring 39 coils, uncompressed 11.25”
AR10 Rifle - Buffer 5.4oz, 5.215” - Spring 34 coils, uncompressed 13.75”
AR10 Carbine - Buffer 5.4oz, 3.26” - Spring 34 coils, uncompressed 13.75”
DPMS 308 Rifle - Buffer 5.3oz, 5.31” - Spring 39 coils, uncompressed 12.75”
DPMS 308 Carbine - Buffer 3.4oz, 2.48” - Spring 27 coils, uncompressed 11.5”
AR15 Carbine Buffer Weights - STD 3.0oz, H 3.8oz, H2 4.6oz, H3 5.4oz
Note - Super special extra heavy buffers (H2, H3) and springs have caused me nothing but problems with anything but full power Nato spec ammo.
AR15 RIfle - Buffer 5.2oz, 5.905” - Spring 44 coils, uncompressed 12.75”
AR15 Carbine - Buffer 3.0oz, 3.25” - Spring 39 coils, uncompressed 11.25”
AR10 Rifle - Buffer 5.4oz, 5.215” - Spring 34 coils, uncompressed 13.75”
AR10 Carbine - Buffer 5.4oz, 3.26” - Spring 34 coils, uncompressed 13.75”
DPMS 308 Rifle - Buffer 5.3oz, 5.31” - Spring 39 coils, uncompressed 12.75”
DPMS 308 Carbine - Buffer 3.4oz, 2.48” - Spring 27 coils, uncompressed 11.5”
AR15 Carbine Buffer Weights - STD 3.0oz, H 3.8oz, H2 4.6oz, H3 5.4oz
Note - Super special extra heavy buffers (H2, H3) and springs have caused me nothing but problems with anything but full power Nato spec ammo.
THE STANDARD DIRECT IMPINGEMENT GAS LENGTHSA general formula for barrel/gas system configuration is: sub-16” barrels should have carbine gas systems, 16”-18” barrels with Mid-Length gas systems, and 18”+ barrels with rifle length systems. Obviously there is some leeway, but a 20” barrel with a carbine system will probably be way over gassed and a 16” barrel with a rifle length system will generally have some under-gassed cycling issues.
Even with a properly designed system an adjustable gas block will help you fine tune the system. Once properly tuned, it will deliver you a stunningly near recoilless and reliable shooting rifle all without adding weight to your action or without changing any other components.
GRIP/DETENT SPRING
Grip Spring & Detent - Almost every AR owner at some point swaps their grip, however nearly every grip swap virgin usually also ends up on all fours looking for the grip spring and detent after pulling off the grip. … and then ends up having a crunchy sounding selector when reinstalled. It should be noted that the grip itself partially holds the spring for the selector switch and the other half of the spring extends up into the selector detent hole pressing against the detent. Watch how it comes apart and prevent either crushing the spring during install or the dreaded boing of the spring across the room.
Grip Spring & Detent - Almost every AR owner at some point swaps their grip, however nearly every grip swap virgin usually also ends up on all fours looking for the grip spring and detent after pulling off the grip. … and then ends up having a crunchy sounding selector when reinstalled. It should be noted that the grip itself partially holds the spring for the selector switch and the other half of the spring extends up into the selector detent hole pressing against the detent. Watch how it comes apart and prevent either crushing the spring during install or the dreaded boing of the spring across the room.
MAGAZINESGenerally I have not had any problems with USA made steel, stainless, or aluminum magazines or with Troy’s and Magpul’s polymer mags. Other mags work well also, however I would note caution as there has been a flood of really crappy mags hit the market which I personally would run from. European mags are less forgiving than US mags as they adhere to a very tight tolerance level. Most 30-round Euro mags, such as E-Lander, are designed for a standard 28-round military spec magazine load to assure positive reloads on a closed bolt.
As any military trained person and they will tell you they only load 28-rounds in a 30-round mag just to assure positive reloads. USA mags in general are designed around the “US consumer” expectations to load a full 30-rounds, however the Euro mags are not. Also euro spec AR15 receivers are less sloppy than to what many US manufacturers adhere to, which in the end means that a Euro mag may be a bit too tight to properly cycle. Be safe and load 28 and you will have far less issues overall with any mag you use.
First and second generation DPMS 308 factory mags are specifically problematic and contribute greatly to feeding and functioning issues. With a few modificaions they can work fine, however in general, I have swapped to Magpul LR-20 magazines which have resolved a great number of 308 related functioning issues.
AMMOSome ammo is shit and will cause feeding and functioning issues. Specifically I have tested and proven to myself that Herter’s 62gr .223 Remington rounds are underpowered. If you want to have a bunch of jams, but this stuff. In most other cases, I have not had any significant issues with any other ammo. My favorite plinking/fun ammo has become the American Eagle 55gr and Hornady Steel case which is sadly temporarily discontinued.
FINAL THOUGHTS
My hope is this prevents some great ARs from being sold or pushed to the back of the safe.
Of course you will need parts and there is no place better to buy than at Brownells.com
My hope is this prevents some great ARs from being sold or pushed to the back of the safe.
Of course you will need parts and there is no place better to buy than at Brownells.com